Back when this software was installed this 'cookie timeout' timer issue was discussed. The problem is no matter what the set timeout threshold, someone can always ask for 'a few more minutes'. It's as easy to walk away from the computer for 30 or 40 minutes as it is for 20.
The basic complaint here is that after 20 minutes if you use the 'Posts since last visit' link it will not show (now by Bold text) threads you have not read because after 20 minutes of no activity, the software assumes you have left thus your 'Last Visit Time and Date' changes and is written to the database.
However, using the Last 24 Hours link will give you all posts and whether you have read them or not - the 'Last Visit' timer is not part of the equation with this search.
I don't know right now. Let me think it over. I might go for 30 minutes, but I want to think about the overall effects. So far as I can see it's minor, all in all. Basically it will skew the 'Visitors Concurrently Online' number causing it to increase and degrades the relevance of the 'Who's Online' function.
Remember, thought, all it takes to reset the timer for another 20 minutes before the software logs you out is to load a forum page or anything else in the forums.